The present invention relates to methods for removing nitrogen oxides effectively from exhaust gas containing the nitrogen oxides such as exhaust gas from internal combustion engines of automobiles etc, exhaust gas from consumers appliance such as cooking apparatus and the like, and exhaust gas from boilers in factories and thermal power stations.
Further, the present invention relates to catalysts for removing nitrogen oxides from the exhaust gas. The catalysts relating to the present invention are preferable for purifying the exhaust gas released from lean burn engines.
Nitrogen oxides (NOx) are contained in exhaust gas released from internal combustion engines of automobiles etc. The nitrogen oxides are harmful to human bodies, and become an origin to destroy a global environment by acid rain etc. Therefore, various catalysts for removing the nitrogen oxides in exhaust gas have been investigated.
Most of catalysts for purifying exhaust gas from automobiles which are under developing at present have been aimed at treating exhaust gas from automobiles wherein a ratio of fuel to air, that is air/fuel by weight, is set at approximately stoichiometric ratio, that is theoretical air to fuel ratio (air/fuel=14.7 by weight). Combustion under the stoichiometric condition generates and releases hydrocarbons and carbon monoxide in addition to the nitrogen oxides. The hydrocarbons and carbon monoxide are also environment contaminating material. Therefore, three way catalyst which can treat the above three substances simultaneously have been a main object of the development. The three way catalyst is a general name for catalysts which can simultaneously treat nitrogen oxides, hydrocarbons, and carbon monoxide in exhaust gas. Most of the three way catalysts contain noble metals such as rhodium, palladium, and platinum as for main components.
However, currently, lean burn engines which burn fuel under a fuel to air ratio (air/fuel) larger than the theoretical fuel to air ratio are becoming a main current as for automobile engines in view of decreasing fuel consumption. With the lean burning, oxygen content in the exhaust gas increases, and an activity of conventional three way catalyst decreases in the presence of the oxygen. Accordingly, nitrogen oxides in the exhaust gas can not be removed effectively. Therefore, development of catalyst for purifying exhaust gas from lean burn engines becomes necessary.
As the catalyst for purifying exhaust gas from lean burn engines, a catalyst which is composed of copper supported by zeolite (JP-A-1-130735 (1989), Proceeding of 68th. Meeting for discussing catalyst, 3F108, (1991)), and a catalyst which is composed of cobalt and rare earth metals supported by zeolite and further at least one of copper and rhodium is supported by the zeolite (JP-A-4-219147 (1992)) are disclosed.
A catalyst which absorbs NOx under a lean burning condition, desorbs the absorbed NOx under a stoichiometric burning condition, and reduces the NOx is disclosed in JP-A-5-261287 (1993). The catalyst is composed of barium oxides, lanthanum oxides, and platinum, all of which are supported by an alumina supporter.
All the above described conventional catalysts for purifying exhaust gas from lean burn engines have such a problem that the catalysts lack a long durability, because all the catalysts contain zeolite. Under the lean burning condition, water is generated approximately 10% by volume by combining hydrocarbons and oxygen in the exhaust gas. Zeolite has such a property that the zeolite loses zeolite structure when it is heated under a condition existing water. Once the zeolite structure is broken, active components supported by the zeolite coagulates, and the catalytic activity decreases.
The catalyst which is composed of barium oxides, lanthanum oxides, and platinum, all of which are supported by an alumina supporter has such a problem that the barium oxides which is contained by a high concentration are thermally deteriorated.
Accordingly, one of the objects of the present invention is to provide a method for treating exhaust gas to purify effectively the exhaust gas released from lean burning.
Other one of the objects of the present invention is to provide a catalyst which can purify the exhaust gas released lean burning effectively, and has preferable durability.
The method for treating exhaust gas relating to the present invention comprises the process of contacting the exhaust gas flow containing nitrogen oxides with the catalyst in the presence of at least one of hydrocarbons and carbon monoxide to reduce the nitrogen oxides to nitrogen.
The catalyst comprises a structure wherein active components are supported by inorganic oxide supporters, the active components comprises at least one of noble metals selected from rhodium, platinum, and palladium, at least one of rare earth metals, at least one of alkali earth metals, and magnesium. Concentration of the noble metal is in a range of 0.05xcx9c3.5 mol % to the inorganic oxides supporter 100 mol %, the rare earth metal is in a range of 0.7xcx9c20 mol %, the alkali earth metal is in a range of 4-16 mol %,.rhodium is less than 1.9 mol %, platinum is less than 2.6 mol %, palladium is less than 2.8 mol %, the noble metal is contained in a form of metal or oxide, the rare earth metal is contained in a form of oxide, and the alkali earth metal is contained in a form of oxide or carbonic acid salt.
As the active component for the catalyst of the present invention, the active component composed of rhodium, platinum, cerium, and magnesium is most preferable, and the catalytic activity is maximum.
The catalyst of the present invention is preferably composed of in a manner that the inorganic oxide supporter supports the rare earth metal, the rare earth metal supports the noble metal, and the noble metal supports the alkali earth metal. The catalyst of the above described structure has a preferable dispersibility of noble metal components and a high catalytic activity.
The noble metal components are composed of rhodium and platinum, and the rhodium is preferably supported on the platinum.
In the catalyst of the present invention, the noble metal gives a reaction field for generating N2 from nitrogen oxide and hydrocarbon. By containing alkali earth metal and/or rare earth metal, adsorption of nitrogen oxide at surface of the catalyst is enhanced. Further, the alkali earth metal and the rare earth metal have a strong bonding activity with oxygen, and accordingly, the metals can absorb oxygen in NOx and proceed the catalytic reaction to form N2 even under a condition existing oxygen.
Magnesium in the active components has an effect to enhance the reducing reaction of the nitrogen oxides by increasing crystallinity of the noble metal. The crystallinity of the noble metal can be enhanced by supporting magnesium after supporting the noble metal on the supporter.
When the catalyst is a mixture of particles composed of an inorganic oxide supporter supporting the noble metal, cerium, and magnesium as the active components and particles composed of an inorganic oxide supporter supporting the noble metal, lanthanum, and barium as the active components, the catalyst has a significantly high catalytic activity under both stoichiometric and lean burning operating conditions. The particles composed of an inorganic oxide supporter supporting the noble metal, cerium, and magnesium have a superior performance for purifying the exhaust gas under both stoichiometric and lean burning conditions, and the particles composed of an inorganic oxide supporter supporting the noble metal, lanthanum, and barium have a property to absorb NOx under the lean burning condition. Therefore, the catalytic performance for purifying the exhaust gas can be enhanced by combining the above two kinds of particles, because NOx is absorbed under the lean burning condition and the NOx is released under the stoichiometric burning condition to be reduced to N2.
As for the inorganic oxide supporter, porous oxides such as TiO2, SiO2, Zro2, MgO and the like can be used. Especially, at least one selected from composite oxide of lanthanum and xcex2-alumina (La. xcex2-Al2O3) and xcex2-alumina (xcex2-Al2O3) is preferably used.
When treating exhaust gas by the catalyst relating to the present invention, it is preferable to make the catalyst contact alternately with a gas flow of low oxygen concentration wherein an oxygen concentration by volume is set in a range of 1.0xcx9c1.7% and a gas flow of high oxygen concentration wherein the oxygen concentration by volume is set higher than that of the gas flow of low oxygen concentration. If the catalyst of the present invention is contacted with the gas flow of high oxygen concentration continuously, an oxide film is generated at surface of the catalyst and activity of the catalyst decreases gradually. Therefore, after contacting the catalyst with the gas flow of high oxygen concentration, subsequently the catalyst is contacted with the gas flow of low oxygen concentration in order to eliminate the oxide film generated at the surface of the catalyst by reacting with hydrocarbon or carbon monoxide. In accordance with the alternate contact with the gas flow of high oxygen concentration and the gas flow of low oxygen concentration, the activity of the catalyst can be maintained at a preferable level for a long time. The time for contacting the catalyst with the gas flow of high oxygen concentration or low oxygen concentration can be respectively from a few tens seconds to a several minutes.
The catalyst of the present invention has preferable performance for purifying exhaust gas under both stoichiometric condition and lean burning condition. Concretely saying, the catalyst of the present invention can reduce nitrogen oxides in the exhaust gas released under the above both conditions to nitrogen effectively by using hydrocarbons and carbon monoxide as reducing agents. Therefore, both hydrocarbons and carbon monoxide can be treated simultaneously.
In accordance with installing the catalyst of the present invention in an exhaust gas system of an internal combustion engine, release of nitrogen oxides to outside the automobile can be suppressed remarkably. Especially, if lean burning and stoichiometric burning are set to be performed alternately, the releasing amount of the nitrogen oxides can be suppressed small for a long time without changing the catalyst.
The catalyst of the present invention is also effective for treating an exhaust gas from diesel engines of diesel automobile and others. The diesel engine is operated with a high air to fuel ratio, that is an oxygen excess condition. However, the catalyst of the present invention has a preferable activity in the presence of oxygen. Therefore, the catalyst of the present invention can remove nitrogen oxides effectively even from an exhaust gas released from a diesel engine.
When cerium is contained in the catalyst of the present invention, the contained amount is preferably in a range from 0.7 mol % to 20 mol % per an inorganic supporter 100 mol %. When magnesium is contained in the catalyst of the present invention, the contained amount is preferably in a range from 4 mol % to 16 mol % per an inorganic supporter 100 mol %.
The catalyst of the present invention can be used in various shapes such as powder, particles, pellets, honeycomb, and others. Furthermore, the catalyst can be used by supported with porous honeycomb such as cordierite honeycomb and metal honeycomb.
The catalyst of the present invention can be prepared by various methods such as impregnation method, kneadering method, coprecipitation method, sol-gel method, and others.
When the catalyst is prepared by the impregnation method, the method comprises preferably the steps of immersing an inorganic oxide supporter into a solution containing rare earth metal compounds, calcining the impregnated supporter, immersing the calcined supporter into a solution containing noble metal compounds, calcining the impregnated supporter, immersing the calcined supporter into a solution containing alkali earth metal compounds, and calcining. In accordance with the above steps, the catalyst can be prepared, wherein the rare earth metals are supported on the inorganic supporter, the noble metals are supported on the rare earth metal, and the alkali earth metals are supported on the noble metals.
As for the metal compounds, various kinds of compounds such as nitrates, acetates, hydrochloride, sulfates, and carbonates can be used.
The catalyst of the present invention has a superior activity in a temperature range from 100xc2x0 C. to 800xc2x0 C., especially has a preferable activity in a range of 200xc2x0 C.xcx9c500xc2x0 C. Accordingly, the temperature whereat the catalyst is contacted with the gas flow, that is a reaction temperature, must be set in the above described temperature range.